A 13% Cr martensitic stainless steel is frequently used in an oil well environment containing carbon dioxide and a small amount of hydrogen sulfide. More specifically, an API—13% Cr steel (13% Cr—0.2% C), which is specified by API (American Petroleum Institute), is widely used since it has an excellent corrosion resistance against carbon dioxide (% used herein means mass % unless a special usage). However, it is noted that the API—13% Cr steel has a relatively small toughness. Although it can generally be used for an oil well steel pipe which normally requires a yield stress of 552 to 655 MPa (80 to 95 ksi), there is a problem that a reduced toughness prevents the steel pipe from being used in an oil well having a much greater depth, since it requires a high yield stress of not less than 759 MPa (110 ksi).
In recent years, modified type 13% Cr steel has been developed in order to improve the corrosion resistance, in which case, an extremely small amount of C content is used and Ni is added instead of the reduced carbon content. This modified type 13% Cr steel can be used in much severer corrosion environments under a condition of requiring a high strength, since a sufficiently high toughness can be obtained. However, such a reduction in the C content tends to precipitate δ ferrites which cause the hot workability, corrosion resistance, toughness and the like to deteriorate. In order to suppress the generation of ferrites, it is necessary to appropriately include expensive Ni in accordance with the added amount of Cr, Mo and other, thereby providing a great increase in the production cost.
Several attempts have been proposed to improve the strength and toughness in both API—13% Cr steel and modified 13% Cr steel. For instance, in Japanese Patent Application Laid-open No. H08-120415, it is shown that an attempt has been made to improve the strength and toughness using effective N which cannot be stabilized by Al on the basis of API—13% Cr steel. In this prior art, however, it follows from the description of the embodiments that steel having a yield stress of order of 552 to 655 MPa (80 to 95 ksi) provides a fracture appearance transition temperature of −20 to −30° C., at most in the Charpy impact test, thereby making it impossible to ensure a sufficient toughness at such a high strength as 759 MPa (110 ksi).
In Japanese Patent Applications Laid-open No. 2000-144337, No. 2000-226614, No. 2001-26820 and No. 2001-32047, a technique for ensuring a high strength and a high toughness in improved 13% Cr steel having low carbon content is respectively described, wherein such a high strength and such a high toughness can be obtained by controlling the precipitation of carbides in grain boundaries and by precipitating residual austenite, along with the effective usage of fine V precipitates. For this purpose, it is necessary to add a corresponding amount of Ni or V, which is very expensive, and further to control the temper condition to a very restricted extent, thereby again providing a great increase in the manufacturing cost, compared with those of API—13% Cr steel.